This project is designed to provide new insight into the mechanisms by which foreign DNA interacts with host chromosomal DNA. Specifically, we are studying the development of bacteriophage Mu in Escherichia coli. This bacterial virus resembles in many key features the animal DNA tumor viruses, DNA insertion sequences and DNA transposition elements. We are focusing our efforts on the following points: 1. Analysis of the early events in the in situ replication of prophage DNA after induction of virus development, including the mechanism for termination of virus specific DNA synthesis at the ends of the prophage DNA. 2. Definition of the replicative form(s) for Mu DNA and cellular location of Mu DNA replicas. 3. Definition of the role of, Hc-DNA - covalently closed circular DNA forms containing both viral and host DNA. 4. Evaluation of the effects of vegetative Mu development upon the integrity and organization of host DNA.